1. Field of the Invention
The present invention relates generally to a fuel concentration monitoring unit for monitoring concentration of particular fuel in a mixture fuel in which different types of fuels are mixed. More particularly, the invention relates to a electrostatic capacity type fuel concentration sensor which is suitable for monitoring concentration of alcohol in a methanol mixed fuel for an automotive internal combustion engine.
2. Description of the Background Art
In the recent days, it is becoming important task to achieve high anti-polution performance in an automotive internal combustion engine. Particularly, reduction of CO.sub.2 gas, NO.sub.x gas from exhaust gas is considered important for future automotive technologies. In this aspect, it has been known that alcohol fuel, such as methanol, is better fuel for smaller emission amount of CO.sub.2 and NO.sub.x. Therefore, in the modern automotive technologies, there have been various attempt to develop automotive engines which can be driven by a mixture fuel of gasoline and alcohol. In such type of internal combustion engine, adjustment of air/fuel ratio and spark ignition timing depending upon alcohol concentration in the mixture fuel is important task in order to optimize engine performance.
Namely, in case of pure gasoline, stoichiometric air/fuel ratio is approximately 14.7:1. In contrast to this, in case of methanol, stoichiometric air/fuel ratio becomes approximately 6.5:1. Therefore, stoichiometric air/fuel ratio varies in substantially wide range depending upon alcohol concentration in the mixture fuel. In fuel injection control for the automotive internal combustion engine, it thus becomes necessary to correct fuel injection amount depending upon alcohol concentration in the mixture fuel. In order to precisely determine an alcohol concentration dependent correction value, it becomes essential to precisely monitor alcohol concentration in the mixture fuel.
In order to monitor alcohol concentration, various types of alcohol concentration monitoring devices have been developed. Typically, the alcohol concentration monitoring devices monitors alcohol concentration by monitoring electric resistance utilizing difference of electric conductivity, by monitoring electrostatic capacity utilizing difference of dielectric constant, or by monitoring reflactive index utilizing difference if reflactive index between the gasoline and alcohol.
In case of the electrostatic capacity type alcohol concentration monitoring device, the alcohol concentration is represented by an electrostatic capacity C.sub.s between a pair of electrodes. Namely, the electrostatic capacity C.sub.s can be expressed by: EQU C.sub.s =.epsilon.S/d
wherein
.epsilon.: dielectric constant PA1 S: area of electrode PA1 d: distance between electrodes PA1 a first sensor disposed within a mixture fuel containing a specific type of fuel in a variable mixture rate, said first sensor producing a first sensor signal indicative of concentration of said specific type of fuel; PA1 a second sensor disposed within said mixture fuel for monitoring temperature thereof, said second sensor generating a second sensor signal indicative of temperature of said mixture fuel; PA1 a circuit incorporated in said unit for generating a fuel concentration indicative signal which varies in a given rate across a predetermined reference value set at a predetermined fuel concentration, said circuit being responsive to said second sensor signal to correct said fuel concentration indicative signal value by varying said given rate depending upon said second sensor signal value; and
As is well known, since the dielectric constant of the mixture fuel increases according to increasing of alcohol concentration in the mixture fuel. Therefore, the electrostatic capacity monitored between the electrodes of the alcohol concentration monitoring device may be considered to represent the concentration of alcohol in the mixture fuel. However, the dielectric constant is also variable depending upon temperature of the mixture fuel. Therefore, in order to precisely monitor the alcohol concentration, it becomes necessary to compensate variation of dielectric constant depending upon the fuel temperature. In the prior proposed systems, fuel temperature sensor comprising a thermister, posistor and so forth, are employed to provide fuel temperature indicative data for obtaining the fuel temperature dependent correction value. Such fuel temperature sensors are disposed in fuel piping to monitor the fuel temperature supplied to fuel injection valves. The fuel temperature sensor is connected to a control unit which performs fuel injection control, to supply the fuel temperature indicative data. The control unit is responsive to this fuel temperature indicative data to derive a fuel temperature dependent correction value for correcting the alcohol concentration indicative data supplied from the alcohol concentration monitoring device. Practically, the fuel temperature dependent correction values are set in a form of map or table in a memory unit of the control unit so that the correction value can be derived by map or table look-up in terms of the fuel temperature indicative data.
As can be appreciated, the correction values to be set in the control unit is variable depending upon the type and specification of the alcohol concentration monitoring devices. This implies that the correction values are to be set with respect to specific alcohol concentration monitoring device. Therefore, when different type or specification of alcohol concentration monitoring device is utilized, all correction values have to be updated adapting to the monitoring device to be used.
Furthermore, in the prior proposed systems, the fuel temperature dependent correction for the alcohol concentration indicative data can be done only when the alcohol concentration monitoring device is combined with the control unit. Therefore, in order to perform inspection, it becomes necessary to combine the alcohol concentration monitoring device with the control unit. Otherwise, during inspection, the temperature of the sample fuel has to be adjusted to be lower than a standard temperature for avoiding fuel temperature dependent error. This clearly requires substantial attention for inspection in the production line.
In addition, since the prior proposed system is required to connect the alcohol concentration monitoring device and the fuel temperature sensor to the control unit, separately from each other. Therefore, extra number of wiring harness becomes necessary to increase the cost and possibility of breakage.